1. Field of the Invention
The present invention relates to a radiofrequency identification (RF-ID) system used for identifying an article having a sub unit mounted thereon and managing the article therethrough. More particularly, it relates to an improved RF-ID system in which the power consumption of a sub unit is reduced and/or the null point phenomenon at a master unit is eliminated.
2. Description of the Related Art
RF-ID systems (or data carrier systems) are applied to, for example, an automobile production line. The RF-ID system comprises master units installed at work portions in the production line and sub units mounted on semifinished automobiles, and communication between the master units and the sub units is realized by radio-frequency (RF) signals. Each sub unit stores an identification code and production data of an automobile to be worked. A semifinished automobile having the sub unit mounted thereon and moving along the production line is identified by the master unit and worked in accordance with the production data stored in the sub unit. After completion of the working, thereat, data in the sub unit is updated and the semifinished automobile is transferred to a next production process, wherein the semifinished automobile is again identified and worked on. In view of the flexibility now required of manufacturing lines, the RF-ID systems are a highly efficient way of managing production lines.
The RF-ID systems also can be used for identifying a person mounting a sub unit as an identification card. More specific RF-ID systems will be described later with reference to the drawings.
Among the problems of the RF-ID system, each such unit is powered by a battery mounted therein, and thus must have a low power consumption, and a master unit suffers from the occurrence of a null (zero) point phenomenon when a sub unit is moved. These problems have not been solved by the prior art RF-ID systems.
JPA No. 54-121093, entitled "Microwave System" and published on Sep. 19, 1979, discloses a moving article identification system which includes an interrogator and a transponder. In this system, to suppress a wave having an adverse affect, due to a Doppler effect, an interrogation wave transmitted from the interrogator and a response wave transmitted from the transponder are different from each other. For example, when the interrogation wave is a vertical polarized wave, the response wave is a horizontal polarized wave, and when the interrogation wave is a right rotation circularly polarized wave, the response wave is a left rotation circularly polarized wave. JPA No. 54-121093, however, does not disclose a method of reducing the power consumption of the sub unit, or a way of eliminating the occurrence of the null point phenomenon.
JPA No. 57-79476, entitled, "Discrimination System" and published on May 18, 1982, discloses an identification system which can be used for identifying a person, for identifying an automobile, and for sensing a speed thereof, etc. In this system, the interrogation transmits only a carrier having a frequency f.sub.0 to a transponder, and the transponder, having a coding modulator, low frequency generators generating low frequencies f.sub.1 and f.sub.2, and a code generator, transmits a response signal having a frequency f.sub.0 .+-.f.sub.1 or f.sub.0 .+-.f.sub.2, using the carrier from the interrogation. Nevertheless, this identification system can be used only for identifying an article, i.e., can not be used in, for example, an automobile production line in which semifinished automobiles are managed through sub units (transponders). In addition, this identification system cannot eliminate the occurrence of the null point phenomenon.
JPA No. 53-18109, described later in detail with reference to the drawings, discloses a circuit construction by which the null point phenomenon is eliminated, but this circuit construction is very complex.